1. Field
The present disclosure relates to a superconducting cable, and more particularly a superconducting cable in which superconducting conductors with each phase are coaxially disposed trebly to configure a core, and an outer cryostat is configured with an aluminum tube having high conductivity to be used as a neutral line.
2. Description of the Related Art
A superconducting cable may transmit a large amount of power in comparison to a general normal conduction cable by using a superconducting wire as a conductor.
For example, Korean Patent Publication No. 10-2011-0091929 discloses a superconducting cable which may be a one phase superconducting cable having a single core and a multi-phase superconducting cable in which several cores with each different electric phases are tied into a single one, or the like, and the multi-phase superconducting cable particularly includes a three phase (A-B-C phase or R-S-T phase) superconducting cable having three cores.
FIG. 1 is a schematic cross-sectional view showing a general three phase superconducting cable.
Referring to FIG. 1, three cores 20 configuring phases (namely, A phase, B phase and C phase) in a cryostat 10 are disposed at three phase superconducting cable 2. In addition, a fluid for maintaining a superconductor in an extremely low temperature state flows in a space 20a between the cryostat 10 and the three cores 20. The three cores 20 are twisted to form a single integrated axis or common axis. The core 20 having such an integrated or common axis shape is commonly called ‘Co-axial’ cable (in this specification, it is called ‘three-core integrated cable’ (three co-axial cable cores in one cryostat)).
In the three-core integrated superconducting cable 2, the cryostat 10 includes an inner cryostat 11 made of a metallic tube, an adiabatic layer 12 made of multi-layered adiabatic materials, and an outer cryostat 13 keeping a vacuum part 10a by a spacer 15, in order from the inner side to the outer side in a radial direction.
In addition, the superconducting cable core 20 of each phase includes a former 21 having a solid shape or a hollow shape forming a coolant channel therein, a superconducting conductive layer 22, an insulating layer 23 and a superconducting shielding layer 24, in order from the center to the outer side in the radial direction.
As described above, the three-core integrated superconducting cable 2 should have the superconducting conductive layer 22 and the superconducting shielding layer 24 for each of three cores 20 with each phase. Since the superconducting wire occupies 90% of the production cost of a superconducting cable, if the superconducting conductive layer 22 and the superconducting shielding layer 24 of the three-core integrated superconducting cable 2 are made of superconducting wires, a large amount of superconducting wires is required, which deteriorates the competitiveness of the product. In addition, since three cores 20 with each phase are arranged in a triangular pattern in a sectional view, the diameter of the entire cable is great, which increases the construction costs for installing the superconducting cable.
In addition, the three-core integrated superconducting cable is short-circuited from the superconducting shielding layer 24 in a terminal connection box and thus plays a role of a neutral line at ordinary time. Moreover, the three-core integrated superconducting cable should be configured to flow a fault current to the superconducting shielding layer 24 during a short time before a circuit breaker or the like operates when a fault such as earth or phase short occurs in a power system. However, the superconducting shielding layer 24 may not bear an abrupt current increase when a fault current is generated since it is made of a thin-film wire with a small sectional area. Therefore, a copper conductive layer is further provided to be used as a neutral line together with the superconducting shielding layer. As described above, since a copper conductor neutral line should be stranded around each of the cores 20 with three phases, the three-core integrated superconducting cable has a complex structure, a complicated manufacturing process and a high production cost.
Meanwhile, Korean Patent Publication No. 10-2008-0000671 discloses a cable (AC cable), which is also called ‘triaxial cable’, in which superconducting conductors with each phase are disposed to coaxially overlap in addition to the three-core integrated superconducting cable where cores with each phase are cooperatively stranded. In this specification, the superconducting cable is called ‘triaxial cable’.
The triaxial superconducting cable disclosed in KR 10-2008-0000671 is configured so that a single core having superconducting conductive layers (power transmission layers) with different phases, which coaxially overlap trebly with the insulating layers being interposed between them, is disposed from the innermost former to the outside in the radial direction. This cable may decrease the diameter of the core, and so the diameter of the entire cable is somewhat reduced in comparison to the three-phase integrated superconducting cable. However, since a neutral line (neutral layer) made of normal conduction metal should be stranded on the outer circumference of the outer insulating layer of the core in order to cope with zero phase current (less than several hundred amperes) and fault current generated by three-phase unbalanced current at ordinary time, the structure and manufacturing process of the triaxial cable are also complicated, and a high production cost is required.
In particular, in a case where a neutral line is disposed at the outside of the core, the neutral line generates heat since zero phase current flows through the neutral line, and the capacity of a cooling device for cooling the heat increases, which also increases the entire system cost. In addition, if fault current of several ten kilo amperes flows into the cable, an electromagnetic force is generated at the neutral line by the fault current, and the electromagnetic force damages the insulating body, thereby deteriorating the reliability of the superconducting cable.